(1) Field of the Invention
The present invention relates to an image formation apparatus such as a printer or a photocopier, and in particular relates to an art of, during an image formation operation, removing residual electrical charge accumulated in a photosensitive body.
(2) Description of the Related Art
In terms of image formation apparatuses using an electrophotographic process, in recent years it has become common to use an image formation apparatus which includes a photosensitive body having a protective layer formed at an outer most surface thereof in order to improve durability of the photosensitive body.
FIG. 16 is a cross-sectional diagram schematically illustrating one example of layer configuration of the aforementioned photosensitive body. As illustrated in FIG. 16, the photosensitive body includes a conductive substrate 3001 which is for example made of aluminum. The photosensitive body also includes an under-coat layer (UCL) 3002, a charge generation layer (CGL) 3003, a charge transport layer (CTL) 3004, and a protective layer 3005, layered in respective order on the conductive substrate 3001. The UCL 3002 is for example made of a resin, the CGL 3003 is for example made of a charge generating material and a resin, the CTL 3004 is for example made of a hole transport material and a resin, and the protective layer 3005 is for example made of a binder resin and inorganic fine particles.
In order to start image formation on the photosensitive body, a negative charge is applied to an outermost surface (i.e., an image carrier surface) of the photosensitive body, as illustrated in FIG. 16. The photosensitive body to which the negative charge has been applied is subsequently exposed to light indicated by dashed arrow A (note that only the leftmost dashed arrow in FIG. 16 is labeled using the reference sign A, whereas reference signs are omitted for other dashed arrows indicating the same). During the aforementioned light exposure, the CGL 3003 receives the light indicated by arrow A and generates, as a pair, a hole P (note that only a leftmost hole in FIG. 16 is labeled using the reference sign P, whereas reference signs are omitted for other holes) and an electron Q (note that only a leftmost electron in FIG. 16 is labeled using the reference sign Q, whereas reference signs are omitted for other electrons).
As illustrated by solid arrow B in FIG. 16 which points in an upward direction (note that only a leftmost solid arrow in FIG. 16 is labeled using the reference sign B, whereas reference signs are omitted for other solid arrows indicating the same), the hole P is attracted towards the negative charge at the outermost surface of the photosensitive body. As a consequence, the hole P moves through the CTL 3004 and the protective layer 3005 to reach the outermost surface of the photosensitive body, and neutralizes the negative charge at the outermost surface, causing formation of an electrostatic latent image. On the other hand, as illustrated by arrow C in FIG. 16 (note that only a leftmost solid arrow in FIG. 16 is labeled using the reference sign C, whereas reference signs are omitted for other solid arrows indicating the same), the electron Q is attracted towards a positive charge in the conductive substrate 3001. As a consequence, the electron Q moves through the UCL 3002 to reach the conductive substrate 3001.
Formation of the protective layer 3005 at the outermost surface of the photosensitive body as described above, improves resistance of the photosensitive body to abrasion, and thus improves durability of the photosensitive body by preventing the surface of the photosensitive body being worn away due to contact, for example with toner or a cleaning member.
However, experiments conducted by the inventors of the present application have confirmed that in a configuration such as described above, in which the protective layer 3005 is layered outward of the CGL 3003 and the CTL 3004, when the hole P moves towards the outermost surface of the photosensitive body, the hole P may become trapped at an interface between the CTL 3004 and the protective layer 3005 as illustrated by dashed circles in FIG. 16.
The trapped hole P eventually moves to the outermost surface of the photosensitive body as a result of repeated performance of image formation and thereby neutralizes negative charge at the outermost surface. As a consequence, an electrostatic latent image which is not related to image data is formed on the photosensitive body and when the electrostatic latent image is developed, image noise (i.e., a so called ghost image) occurs on a recording sheet which is undergoing a printing process.
In terms of an art for preventing the aforementioned image noise, Japanese Patent Application Publication No. 2013-7813 for example discloses an art of, during a period in which printing jobs are suspended, applying an electrical charge to an image carrier surface of a photosensitive body, while rotating the photosensitive body at least once, in order to remove residual charge (holes) trapped in the photosensitive body.
Through the art described above, it is possible to prevent image noise occurring even when an image formation operation is performed repeatedly.
Unfortunately, in the art described above, an operation of removing residual charge from the photosensitive body by applying electrical charge is performed while printing jobs are suspended. Therefore, the art described above suffers from a problem of being unable to prevent image noise, occurring due to accumulation of residual charge in the photosensitive body, during performance of an image formation operation. In consideration of the above problem, in order that the operation of removing residual charge described above is performed during the image formation operation, the removal operation could be performed in a period between successive recording sheets, which in other words corresponds to a period between completion of light exposure scanning based on image data of a preceding page and commencement of light exposure scanning based on image data of a next page after the preceding page. Unfortunately, in order to perform the removal operation during the aforementioned period, the period is required to be long enough for the photosensitive body to rotate once, resulting in a problem of reduced image formation productivity.